This invention relates to the recording of optical signals and particularly to an improved optoelectronic recording medium and a method of making that medium.
The recording of optical signals, particularly in large volume and at a high rate, is usually accomplished either by indirect means or by direct means.
In the indirect mode of recording, the optical signal, e.g., a light wavefront, is received by an array of CCD's or other sensors and converted to electronic signals which, in turn, produce magnetic signals to be recorded on a magnetizable medium such as a magnetic tape or disc.
The direct method of recording, on the other hand, involves a direct interaction between the light signal, after it has been properly focused, shaped and geometrically arranged, and a light sensitive medium for direct storage. In this latter method, the storage medium is usually photographic film or a photoconductive material such as selenium or zinc oxide dispersed in a dielectric binder. The former type of medium is used mostly in photographic cameras, while the photoconductive medium is incorporated into office copiers.
The advantages of indirect recording include the ease of reading and processing the converted signal. That signal, recorded magnetically in serial fashion, is readily compatible with electronic circuits that can manipulate and process the recorded information. Another advantage of the indirect mode of recording is the ease with which the information can be erased either partially or totally. In other words, in an indirect recording system, the optical signals, after having been received and converted to magnetic form, possess the ease of handling which produce the flexibility inherent in magnetic read/write/erase systems. The principle disadvantages of the indirect recording method include signal distortion introduced during signal conversion, the need to switch to a serial information handling format, the relatively low upper limit of the bandwidth of the captured data stream, the relatively poor signal-to-noise ratio of the recording medium and the relatively low packing density of the data stored on the medium, i.e., the large volume of tape or space required to store the original data stream.
The principle advantage of direct optical recording is the ease with which the incoming optical signal stream can be routed to the recording medium. The raw information is captured in analog form and stored in a parallel manner so as to retain the geometric relationships of all of the resolution elements contained in the incoming optical wavefront. However, conventional photographic recording techniques have several disadvantages which seriously limit their applications. These include low efficiency during processing in the conversion of the light signal to an ionic chemical signal on the film, the failure to achieve energy reciprocity at signal durations faster than the microsecond range, the need to process the acquired optical signal chemically in order to fix it to the film and the difficulty in accomodating the acquired signal to match the needs of standard electronic data processing circuitry.
Direct recording using known optoelectronic or photoconductive media does not involve chemical processing. In this respect, then, it is preferable to photography, prompting industry to devote considerable resources to improve this mode of data recordation. The efforts in this regard have led to the development of a variety of direct recording optoelectronic film and plate structures. The ones that show the most promise comprise a photoconductive light modulating section and a dielectric storage section. By exposing the modulating section to a light image, an electrical charge can be impressed on the storage section whose spacial distribution over the area of the storage section is an electrical analog of the original image.
In one medium of this type, described in U.S. Pat. No. 2,825,814 (Walkup), the light modulating section and the storage section are separate structures which are assembled in use. That is, the modulating section comprises a photoconductive layer with a transparent conductive base and the storage section is a dielectric layer with a transparent conductive base. In use, the photoconductive and dielectric layers are placed in contact and a high voltage is applied between the conductive bases of the two sections, while a light image is projected onto the assembly. After a brief period, the light is turned off and the two members are separated leaving the light image stored on the dielectric layer as an electrical charge distribution. The image can then be developed by applying toner to that section. This type of recording medium is disadvantaged in many respects. These include the requirement of a high charging voltage with its attendant danger, the necessity of assembling and disassembling the modulating and storage sections and the distortions in the image-representing electrical charge on the dielectric layer due to the air gap inevitably present between the assembled sections.
Another type of recording medium which does not involve such assembly and dissassembly of the modulating and storage sections of the medium is described in Electrostatic Imaging and Recording by E. C. Hutter et al., Journal of the S.M.P.T.E., Vol. 69, January 1960, pp. 32-35. This medium has a transparent organic plastic base layer, such as polyester film, coated on one side with a layer of photoconductive material which is, in turn, coated with a thin layer of a dielectric material. To record an image on the medium, the dielectric layer is precharged by a corona discharge directed to that layer. Then, the photoconductive layer is exposed to a light image, while an electric field is applied across the dielectric layer. The charge in the dielectric layer decays towards zero with the decay being most rapid where the optical image is brightest and, therefore, the photoconducter resistance the lowest. After a time corresponding to the greatest difference between the potentials in the light and dark areas of the medium, the electric field is turned off and the discharging process stops thereby leaving on the dielectric layer an electrostatic charge image corresponding to the optical image incident on the medium. The stored image may be developed by applying toner to the medium or it may be read from the medium by scanning the dielectric layer with a focused electron beam as is done in a vidicon tube to produce a capacitively modulated electrical signal corresponding to the stored image. While this medium is a unitary structure, a voltage must be applied to the medium prior to exposure in order to precharge the dielectric storage section. This increases the cost and complexity of the associated recording apparatus. Also, the image-representing current signal produced by such scanning has relatively poor quality and low signal-to-noise ratio. Furthermore, that scanning process requires a source of high voltage making that medium impractical for use in a portable self-contained instrument such as a microscope or camera which relies on battery power. The medium has several other disadvantages as well which seriously limit, if not prevent, its practical application. More particularly, it has poor light sensitivity comparable to the slowest silver halide films. Furthermore, it can store the acquired data only for a limited period of time, e.g., a few weeks, because of charge leakage in the dielectric storage layer of the medium. Furthermore, that medium is not physically strong or rugged enough to be practical for long-term information storage. U.S. Pat. No. 3,124,456 (Moore) shows a similar structure that is similarly disadvantaged.
Another type of multi-layer electrostatic storage medium which does not require precharging of the medium is disclosed in U.S. Pat. Nos. 4,155,640 and 4,242,433 to Kuehnle et al. This medium comprises a transparent plastic substrate or base which carries a layer of photoconductive material, there being a conductive layer between the photoconductive layer and the base. Superimposed on the photoconductive layer is a layer of dielectric material and on top of that is another conductive layer completing the sandwich. In operation, a low DC voltage is applied to the sandwich between the two conductive layers while the medium is exposed to a light image through the transparent base. The light image causes the photoconductive layer to modulate the flow of charge carriers so that an electrostatic image is impressed on the dielectric storage layer. Thereafter, the conductive layer adjacent the storage layer is stripped off so that a charge distribution corresponding to the original light image remains on the dielectric layer. The stored image can be developed by toner or read by electron beam scanning. While that medium is a unitary structure, it does require the removal of the electrode layer from the storage section following exposure in order for the image-representing charge to remain on the medium. This strippable conductor necessitates the presence of a conductive fluid or a fusible bonding layer between the conductor and the dielectric layer in order to obtain the necessary intimacy between the electrode and the dielectric. This complicates the manufacture of the recording medium and, in the case of the fusible bonding layer, it requires the presence in the associated camera or recorder of a hot shoe or similar device to melt the bonding layer to permit removal of the conductor. That medium also is characterized by the presence of so-called dark currents in its photoconductive layer which result in charge leakage from the dielectric layer. This makes that medium unacceptable for signal storage over an extended period of time.
Yet another recording medium disclosed in U.S. Pat. No. 3,880,514 (Kuehnle) avoids the requirement of a removable conductor to store an image on the medium. However, this is done by eliminating the dielectric layer from the medium. Accordingly, that film can only store an image for a short time due to charge leakage through its photoconductive layer.
Additional problems affecting all of the prior electrographic recording media of which we are aware, including the phototapes and films specifically discussed above, stem from the fact that the materials in all of those multi-layer structures are selected primarily for their ohmic electrical properties and general commercial availability, with minimal consideration being given as to how the various layers should be integrated into a total overall structure which would achieve unprecedented performance. In fact, the layers in the prior structures are made without attention to the interrelationship and the compatibility of those layers. As a result, there are definite mechanical boundaries between the adjacent layers of the media which are a source of internal electrical noise and inconsistencies. Also, various layers may differ in their degrees of perfection giving rise to poor sensitivity, a high noise level in the stored image and premature loss of that image.
Most critically, the importance of the substrate or base material in influencing dramatically the overall operation of the recording medium has been totally overlooked in the prior media. That is, electrographic tapes and films such as those described above, usually utilize for the base a polyester or other organic plastic material. Made as a thin film or tape, this material is quite strong and flexible; also, it is optically clear, at least initially. However, it is subject to elongation and distortion making it difficult to achieve a good bond or adherence of the light modulating section of the medium to the base. This problem can be alleviated to some extent by including a special bonding layer between the substrate and the medium's modulating section as discussed in U.S. Pat. No. 4,269,919 (Kuehnle). On the other hand, that solution creates additional interfaces and boundaries in the medium which are undesirable, as noted above. It also increases the complexity of the medium and the cost of making it since the formation of each layer in the medium involves a separate sputtering or coating process. Still further, while the plastic substrates of the prior flexible tapes and films may have excellent optical clarity when the medium is new, as soon as the medium is placed into service, its substrate reacts to the incident energy at the ultraviolet end of the light spectrum by losing its optical clarity, making the medium less responsive to low light energy levels These plastic substrates are not particularly scratch resistant either, so that the substrate surfaces often have scratches which impair the medium in the same way.
All of those prior media discussed above with plastic substrates or other components are disadvantaged also because such organic material invariably suffers outgassing when the medium is placed in a vacuum. Bearing in mind that information should ideally be retrieved from these media by electron beam scanning in a vacuum, it becomes apparent that such outgassing will interact with the electrons in the scanning beam and adversely effect, to the point of commercial impracticality, the image-representing electrical signals produced by the scanning process.
To avoid problems caused by such plastics, it has been proposed to make the medium substrate out of an inorganic material such as metal or glass. However, those materials are quite stiff, opaque or fragile. Even if monocrystalline wafers of silicon or sapphire were used, such as those available from the integrated circuit industry, one would face major problems. This is because in order to make such inorganic structures thin enough to be of use for applicant's purposes in an optoelectronic medium, they must be ground and polished to such an extent that there is an excessive amount of breakage. Furthermore, those wafers that do survive the finishing process have surface defects and abrasions caused by such finishing that degrade the bond with, and initiate defects in, any layer of material that is added to the surface of that structure. These internal defects, in turn, reduce the purity and performance of the resultant film to the point of making it useless and impractical as a recording medium for an optoelectronic camera or recorder.
In general, then, while the prior electrographic recording media and processes may work in principle, they are not satisfactory in practice and have never found commercial use. It should be understood in this connection that a suitable recording medium, for applicant's purposes, must be able to be erased completely and also be used a multiplicity of times without any appreciable loss of its strength, flexibility, optical sensitivity or its data storage capability. To applicant's knowledge, none of the known recording media, including those described in the above-identified publications, possess these capabilities and, therefore, none are suitable for the detection and recording of low energy optical signals and for the required long-term storage of equivalent electrical signals which are necessary to obtain the above-stated advantages of both direct and indirect recording.